1. Field of the Invention
The present invention relates to a process for preparing epoxysilanes by reacting a hydrogensilane with an allyl glycidyl ether in the presence of a catalyst followed by purification.
2. Background of the Invention
The preparation of epoxysilanes by reacting hydrogensilanes with allyl glycidyl ether in the presence of a hydrosilylation catalyst based on Pt(0) is described in the German patent application 198 05 083.6. However, other catalyst systems can in principle also be used for hydrosilylation reactions, for example simple compounds or complexes of nickel, platinum, rhodium or ruthenium in their various oxidation states, cf., for example, EP 0 262 642 A2, EP 0277023 A2, EP 0288286 A2 and EP 0548 974 A1.
It is also known that, for example, in the reaction of allyl glycidyl ether with trimethoxysilane, 
the resulting crude product contains not only 3-glycidyloxypropyltrimethoxysilane (GLYMO) 
but also unreacted starting materials, isomerized allyl glycidyl ether, isomerized epoxysilane, viz. 2-glycidyloxy-1-methylethyltrimethoxysilane (iso-GLYMO) 
tetraalkoxysilane and xe2x80x9ccyclic alkoxysilanexe2x80x9d formed by cyclization of epoxysilanes, viz 1-dimethoxysila-2,5-dioxa-3-methoxymethylcyclooctane (cyclo-GLYMO) 
but also the catalyst used.
The crude product is customarily worked up by means of column distillation. The work-up is generally very complicated and time-consuming. Furthermore, it is found that additional amounts of the xe2x80x9ccyclized alkoxysilanexe2x80x9d (cyclo-GLYMO) are continually formed during the distillative work-up.
Since there is only a small difference in the boiling points of the target product xe2x80x9cepoxysilanexe2x80x9d and the xe2x80x9ccyclized epoxysilanexe2x80x9d, the running time of a batch distillation is increased significantly and quantitative removal of the xe2x80x9ccyclized epoxysilanexe2x80x9d by means of column distillation is not possible on an industrial scale. However, quantitative separation of the xe2x80x9ccyclized epoxysilanexe2x80x9d from the main product is necessary to achieve very high epoxysilane purities. In addition, a high proportion of undesirable high boilers, which reduces the yield of epoxysilane, is formed under the conditions prevailing in the distillation.
It is therefore an object of the invention to provide a process for preparing epoxysilanes which avoids the disadvantages discussed above, where possible.
It has surprisingly been found that the abovementioned disadvantages can be avoided if the work-up of the crude epoxysilane product is carried out in two steps, with the catalyst still present in the crude product obtained directly from the epoxysilane synthesis being removed from the crude product in a first step and the crude product mixture which has been essentially freed of catalyst being subjected to a work-up by distillation in a second step.
Thus, the present invention provides a process for preparing an epoxysilane, comprising:
reacting a hydrogensilane with an allyl glycidyl ether in the presence of a catalyst to form a crude product,
removing the catalyst from the crude product, and then
distilling the crude product.
The work-up by distillation of the essentially catalyst-free crude product is preferably carried out by means of column distillation. The removal of the hydrosilylation catalyst from the crude epoxysilane product is preferably carried out by adsorption or by reduction. The present process provides epoxysilanes having high purities.
A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description.
In the process of the invention, the hydrogensilane component used can be, for example, trimethoxysilane, triethoxysilane, methyldimethoxysilane, methyldiethoxysilane, tri-n-propoxysilane, tri-n-butoxysilane, triisopropoxysilane, methyldi-n-propoxysilane, methyldiisopropooxysilane, methyldi-n-butoxysilane, triisobutoxysilane, methyldiisobutoxysilane, phenyldimethoxysilane, phenyldiethoxysilane, phenyldi-n-propoxysilane, phenyldiisopropoxysilane, phenyldi-n-butoxysilane or phenyldiisobutoxysilane. Mixtures of these hydrogensilanes may be used.
In the process of the invention, the preferred epoxysilanes are represented by the general formula I: 
where
R is a linear or branched alkyl group having from 1 to 4 carbon atoms or an aryl group having from 6 to 12 carbon atoms,
R1 is a linear or branched alkyl group having from 1 to 4 carbon atoms, and n is 0 or 1 or 2.
Examples of these epoxysilanes include 3-glycidyloxypropyltrimethoxysilane (GLYMO), 3-glycidyloxypropyltriethoxysilane (GLYEO), 3-glycidyloxypropyltri-n-propoxysilane, 3-glycidyloxypropyltriisopropoxysilane, 3-glycidyloxypropyltrin-butoxysilane, and 3-glycidyloxypropyltriisobutoxysilane.
The hydrosilylation catalyst used in the process of the invention is preferably a noble metal catalyst based on Pt, Pd, Rh or Ir.
The hydrosilylation reaction may be conducted under conditions which are known per se. Then, according to the invention, the catalyst is removed from the product obtained, generally to concentrations of  less than 10 mg/kg, calculated as noble metal, and the essentially catalyst-free crude product is worked up by distillation, for example by means of column distillation.
The catalyst is preferably removed from the crude product by adsorption. The adsorption is advantageously carried out in a fixed absorber bed. However, it is also possible to add a solid absorber to the crude product to be treated and subsequently separate it off by filtration or by means of another liquid/solid separation process.
Preference is given to using adsorbers having a mean particle size of from 1 to 30 mm, particularly preferably from 5 to 20 mm. These ranges include all specific values and subranges therebetween, such as 2, 8, 10, 12, 15 and 25 mm. The adsorbers used in the process of the invention generally have an internal surface area of from 10 to 1500 m2/g. These ranges include all specific values and subranges therebetween, such as 25, 50, 100, 250, 500 and 1000 m2/g.
In particular, it is possible to use activated carbon, silica, for example pyrogenic or precipitated silica, aluminum oxide, titanium dioxide, zirconium oxide or zeolites or else polymeric resins as adsorbers. However, it is also possible to use other materials which are suitable as adsorbers. In the present process, it is also possible to use a mixture of solid adsorbers. It is advantageous to use neutral adsorbers. Furthermore, the absorbents used in the process of the present invention should be essentially free of water.
In the process of the invention, preference is given to using adsorbers having an internal surface area (BET) of from 150 to 1400 m2/g, more preferably from 600 to 1400 m2/g, even more preferably from 1000 to 1500 m2/g. These ranges include all specific values and subranges therebetween, such as 250, 500, 750 and 1200 m2/g.
In the process of the invention, the adsorption is preferably carried out at a temperature in the range from 0 to 120xc2x0 C., particularly preferably at a temperature in the range from 10 to 50xc2x0 C., very particularly preferably at a temperature in the range from 10 to 25xc2x0 C. These ranges include all specific values and subranges therebetween, such as 20, 30, 60, 80 and 100xc2x0 C.
The work-up according to the invention of the said crude product can be carried out under reduced pressure. The adsorption in the process of the invention is preferably carried out at a pressure in the range from 0.5 to 5 bar abs. (absolute), particularly preferably from 0.7 to 2 bar abs. (absolute), very particularly preferably from 1.0 to 1.5 bar abs. (absolute).
The residence time of the crude product in the absorber bed or over the adsorbent is preferably from 5 minutes to 2 hours; preference is given to a contact time of from 10 to 60 minutes. The time is particularly preferably from 15 to 45 minutes.
In the process of the invention, the catalyst can also be precipitated from the crude product by reduction and thus be removed from the crude product. Examples of reducing agents which can be used are metallic zinc in the form of powder or of pieces which can easily be removed from the crude product again.
The inventive process provides the following advantages:
achievement of particularly high epoxysilane purities, in particular xe2x89xa799%, in the work-up by distillation, because the further formation of the xe2x80x9ccyclized epoxysilanexe2x80x9d (=xe2x80x9ccyclic alkoxysilanexe2x80x9d) is very substantially suppressed during the distillation.
shortening of the intermediate fraction in the work-up by distillation and thus an increase in the distillation capacity,
a low proportion of high boilers which are formed during the distillation and thus an increase in the yield of epoxysilane in the distillation,
the adsorbed noble metal catalyst can be worked up to recover the noble metal.
no epoxysilane losses as would occur in the work-up of the crude epoxysilane mixture by means of a thin-film evaporator,
no addition of auxiliaries to deactivate the catalyst is necessary, and
low energy consumption and technical simplicity in removal of catalyst by adsorption.